Low carbon steel



Patented Sept. 3, 1935 No Drawing. Application October 24, 1932, Serial No. 639,382

5' Claims. (01. 148-18) f This invention relates to ferrous metals, and more particularly to the production of a new low carbon steel.

It is well known to the art that carbon steels, meaning thereby steels containing carbon with or without the addition of manganese, but without additions of nickel, chromium, vanadium or other elementsaflectingthe temperature characteristics of the steel, fall into two classes: Tempering steels and non-tempering steels. Tempering steels have a carbon content above about 0.4% carbon. These steels are characterized by the fact that on quenching in watersuch steels show a marked increase in tensile strength. At the same time they become hard and brittle and capable of maintaining magnetism. Examina- ,tion under the microscope shows that such steels after quenching are largely martensitic in structure.

Steels without amounts of alloying elements I I sufilcient to affect the tempering characteristics, with or without manganese up to 0.5%, with carbon ranging from 0.05% to about 0.40%, with or without small amounts of silica and with or without included slag, are known in the art as low carbon steels and are characterized by the fact that quenching in water accomplishes no important increase in tensile strength. Low carbon steels after quenching in water do not have any large proportion of martensitic structure and do not retain magnetism.

The advantages of low carbon steels are that they may readily be worked and cold drawn. The disadvantages are that they are soft andhave low tensile strength.

One of the purposes of my invention is to produce' a steel having the desirable qualities both of high tensile strength, and of great toughness, ductility, and ability to be drawn and worked.

Other purposes of my invention will appear from the following.

I have invented a new class .of low carbon steels characterized by their possessing a large portion of martensitic structure and possessing tensile strength on the order of twice that usual with low carbon steels of the same-chemical composition but not martensitic structure. I have found that such steels may possess in addition the desirable properties of ductility, low fatigue, toughness, ability to be drawn and otherwise cold worked, and ability to be machine worked, sawn and worked with a file. Such properties are known in alloy steels containing such expensive additions as vanadium, chromium and nickel and the like but they are novel for steels containing only a low percentage of carbon with or-v without the additionof manganese, these being the ordinary low carbon steels as that term is used herein.

I have discovered that steels possessing such properties can be produced in several ways of which the following is preferred. Starting with low carbon steel of the appropriate chemical com- .position, that is to say having from 0.05% to 0.40% carbon and up to 0.5% manganese, these steels being any of the class of steels known as cold rolled steel or reinforcing steel or structural steel or furniture steel or deep drawing iron or the like, I may or may not form the material into the form desired, or approximately into the form desired. I then heat the steel to a temperature ranging from red heat to near the melting point of the steel. Preferably I heat the steel to between 900 Fahrenheit and 1650 Fahrenheit in accordance with the carbon content of the steeL. I find it preferable to heat steels of a low carbon content to a higher temperature than steels of a higher carbon content. I then suddenly quench the hot steel in a solution having the power to quench so quickly as to inhibit the transition from gamma iron to other forms of iron.

I have found that the quenching media known to the art of quenching high carbon steels do not in general have the property of quenching low martensitic structure. I presume that this is in part due to the fact that such quenching media do not cool the steel fast enough. I have observed that during the process of quenching steels with the usual media a piece of steel having a thickness of more than one-eighth of an inch remains red hot for several seconds, presumably because it is protected from perfect contact with the liquid media due to the film of vapor which envelopes the steel, and also, perhaps, because of the layer of scale which is formed. Using the quenching media preferred I have observed that a piece of steel cools below red heat within less than one-third of a second. Such steel when removed from the quenching bath is scale free.

' I have found that various quenching materials may be used but that concentrated solutions of 1 alkali hydroxides, especially potassium hydroxide, are preferable since they remove scale and produce the greatest increase of tensile strength. While improved characteristics may be imparted to the steel by using such solutions of as low as 10% concentration, I prefer to use considerably higher concentrations, on the order of 40% solutreating can be applied to the completely formed tions, depending somewhat on the composition and on the characteristics of the steel desired. This quenching material can be used at room temperature, and is preferably maintained at a temperature not much, if any, over 125 F. Low carbon steel so produced is found under the microscope to be largely martensitic in structure and to possess a tensile strength in excess of 100,000 lbs. per square inch, and on the order oftwo or more times the tensile strength of said steel before treatment. At the same time the steel so treated is very tough and ductile, can be cold worked, machine worked, sawn, filed, etc.,

' and is capable of being permanently magnetized.

Other quenching materials which operate to produce steel of improved character, but not the equal of steel produced by quenching in concentrated solutions of alkali hydroxides, are con-- centratedsolutions of very soluble inorganic substances, especially concentrated solutions of deliquescent salts, particularly when kept at low temperatures, on the order of 20 centigrade.

. desired and of the compositions specified to travel in a continuous manner into a region where it is suddenly contacted with a quenching solution of the type specified, which may for example be a 40% caustic potash solution. This contacting may be accomplished by submerging the steel in a bath containing the quenching solution or conveniently by spraying the steel with a high velocity jet or jets of the specified quenching solution.

The present invention also enables articles to be partly shaped or formed from low carbon steel and then treated in accordance with the present invention, after which, owing to the ductility, workability,- etc., of the steel, the shaping or forming may be completed, but if preferred the article.

While the procedure herein described .is preferred, it is to be understood thatother procedures may be adopted within the scope of the claims which follow.

What is claimed is:

1. The process of producing martensitic low carbon steel which includes heating pre-formed low carbon steel to at least red heat and quenching in an aqueous solution of alkali metal hydroxide having a concentration on the order of 2. The process of producing martensitic low carbon steel which includes heating pre-fabricated low carbon steel to at'least red heat and quenching in an aqueous solution of an alkali metal hydroxide having a concentration on the order of 40% and maintained at a temperature below the boiling point of water.

'3. The process of producing martensitic low carbon steel which includes heating pre-formed low carbon steel to at least red heat and quenching in an aqueous solution of potassium hydroxide having a concentration on the order of 40%.

4. The process of continuously producing low carbon steel which is largely martensitic in structure, which includes heating progressively a continuous length of ordinary low-carbon steel to at least red heat and then suddenly cooling the same progressively with an aqueous solution of an alkali metal hydroxide having a concentration on the order of 40%.

5. The process of forming articles of low carbon steel which includes partly forming or shaping the article, heating the partly finished article to at least red heat, cooling the same in an aqueous solution of an alkali metal hydroxide having a concentration on the order of 40%, and then working the treated steel to complete the article.

GEORGE F. NELSON. 

